Synaptic strength must be regulated in the face of changing levels of input in order to ensure that total strength of all inputs are controlled so as to maintain output firing within reasonable limits and these mechanisms must work in conjunction with mechanisms of synaptic plasticity. Its clinical importance can be best illustrated by the condition of epilepsy where these mechanisms are disrupted at the extreme. The association of epilepsy with numerous forms of inherited disorders of cognitive function such as Fragile X and Down Syndrome and the increasing use of anti-epileptic drugs for the treatment of mood disorders highlight the importance of these mechanisms in maintaining normal brain function. This grant proposes to study the underlying mechanisms by which synaptic adaptation is achieved in the face of changing levels of input and the effects of this adaptation on plasticity, building on recent progress in the lab. The first two parts will use strategies to describe more completely the signaling events in the pathway from synaptic input to synaptic modification, taking advantage of the ease of manipulation and measurement in the primary culture system. This part of the study will begin with a careful dissection of the sources of electrical input and calcium entry responsible for the induction of the key signaling events, followed by an elucidation of the mechanisms of regulation of key intermediate signaling products, examining the regulation of their enzymatic activity, stability, transcription, translation and transport to the synapse. In addition, a more complete description of the signaling cascade will be drawn out from microarray studies comparing results from disruption of the signaling pathway at various points. The next part of the study will look closely at the microphysiology of this phenomenon (a) to elucidate the elements of adaptation that are cell-autonomous, by comparing effects of network changes and manipulations in only one cell (b) to describe adaptive changes at the level of single synapses, and (c) to determine the pre- or postsynaptic locus of induction of the various elements of adaptation. These studies will take advantage of long standing expertise in the lab in studying the physiology of single synapses involving focal stimulation and measurement of synaptic responses at unitary sites. The last part of the project will apply the knowledge gained in the previous sections to try to understand the impact of adaptation m intact hippocampal circuits, using the organotypic slice culture. Primary questions addressed will be differences in the expression of adaptation at two different sets of afferents and consequences of this adaptation on the induction and expression of LTP. We will look for forms of LTP not previously found in pyramidal neurons and for changes in the performance of preexisting mechanisms of LTP.